Determining a location of a mobile device

ABSTRACT

It is provided a method for determining a location of a mobile device. The method comprises the steps of: obtaining a first maximum distance between a first anchor point and the mobile device; generating a first circular geometrical object having a radius based on the first maximum distance; generating a first polytope encompassing the first circular geometric object, wherein all angles of the first polytope are right angles; obtaining a second maximum distance between a second anchor point and the mobile device; generating a second circular geometrical object having radius based on the second maximum distance; generating a second polytope encompassing the second circular geometric object, wherein all angles of the second polytope are right angles; finding a mobile device region as an overlap between the first polytope and the second polytope; and determining that the mobile device is located within the mobile device region.

TECHNICAL FIELD

The invention relates to a method, a location determiner, a computerprogram and a computer program product for determining a location of amobile device.

BACKGROUND

Determining a location of a mobile device, such as a mobile phone or keyfob, can be achieved in many different ways. For instance, the locationof the mobile device can be determined by measuring the distance fromtwo or more known points and determining the mobile device to be locatedat a point satisfying the distance measurements. The accuracy is notperfect, whereby there is a margin of error region around the estimatedlocation.

In most situations, it is acceptable that there is a certain margin oferror around an estimated location. However, when the location of amobile device needs to be evaluated with respect to a target region, thepresence of a margin of error is not optimal. For instance, the marginof error region is both inside and outside the target region, it isimpossible to determine whether the mobile device is inside or outsidethe target region. Moreover, even if the mobile device most likely islocated within the margin of error region, this cannot be guaranteed. Inother words, even if a margin of error region is completely inside atarget region, there is a very small risk that the mobile device isactually outside the margin of error region and can thus be outside thetarget region.

US 2007/0268138 A1 discloses object a monitoring, locating and trackingsystem, and a method employing RDID devices. US 2013/0329581 A1discloses a method for positioning a mobile device in a wireless widearea network. US 2014/0073351 A1 discloses determining the location ofmobile terminals in the presence of a repeater. US 2013/0178235 A1discloses a wireless communication positioning method.

SUMMARY

It is an object to provide a way to determine a mobile device region inwhich it can be guaranteed that a mobile device is located.

According to a first aspect, it is provided a method for determining alocation of a mobile device. The method is performed in a locationdeterminer and comprises the steps of: obtaining a first maximumdistance between a first point first anchor point and the mobile device;generating a first circular geometrical object, centred on the firstanchor point, having a radius based on the first maximum distance;generating a first polytope encompassing the first circular geometricobject, wherein all angles of the first polytope are right angles;obtaining a second maximum distance between a second anchor point andthe mobile device; generating a second circular geometrical object,centred on the second anchor point, having radius based on the secondmaximum distance; generating a second polytope encompassing the secondcircular geometric object, wherein all angles of the second polytope areright angles; finding a mobile device region as an overlap between thefirst polytope and the second polytope, and determining that the mobiledevice is located within the mobile device region.

At least one side of the first polytope and at least one side of thesecond polytope may be parallel.

The sides of the first polytope and the second polytope may be alignedwith axes of a Cartesian coordinate system.

The method may further comprise the step of: determining that the mobiledevice is located within a target region when the target region fullyencompassed the mobile device region.

Each side of the first polytope may touch the encompassed first circulargeometrical object and each side of the second polytope touches theencompassed second circular geometrical object.

The geometrical objects may be two dimensional objects, and the methodfurther comprises the step of: finding intersection points between thefirst circular geometrical object and the second circular geometricalobject; and determining which, if any, of the intersection points thatcan be applied to restrict where the mobile device is located, and, whenapplicable, the step of finding the mobile device region comprisesreducing the size of the mobile device region based on the at least oneapplicable intersection point.

The method may further comprise the steps of: obtaining a third maximumdistance between a third anchor point and the mobile device; generatinga third circular geometrical object, centred on the third anchor point,having a radius based on the third maximum distance; and generating athird polytope encompassing the third circular geometric object, whereinall angles of the third polytope are right angles; wherein the step offinding an overlap comprises finding the overlap between the firstpolytope, the second polytope and the third polytope.

According to a second aspect, it is provided a location determiner fordetermining a location of a mobile device. The location determinercomprises: a processor; and a memory storing instructions that, whenexecuted by the processor, cause the location determiner to: obtain afirst maximum distance between a first anchor point and the mobiledevice;

generate a first circular geometrical object, centred on the firstanchor point, having a radius based on the first maximum distance;generate a first polytope encompassing the first circular geometricobject, wherein all angles of the first polytope are right angles;obtain a second maximum distance between a second anchor point and themobile device; generate a second circular geometrical object, centred onthe second anchor point, having radius based on the second maximumdistance; generate a second polytope encompassing the second circulargeometric object, wherein all angles of the second polytope are rightangles; find a mobile device region as an overlap between the firstpolytope and the second polytope; and determine that the mobile deviceis located within the mobile device region.

At least one side of the first polytope and at least one side of thesecond polytope may be parallel.

The sides of the first polytope and the second polytope may be alignedwith axes of a Cartesian coordinate system.

The location determiner may further comprise instructions that, whenexecuted by the processor, cause the location determiner to: determinethat the mobile device is located within a target region when the targetregion fully encompassed the mobile device region.

Each side of the first polytope may touch the encompassed first circulargeometrical object and each side of the second polytope may touch theencompassed second circular geometrical object.

The geometrical objects may be two dimensional objects, and the methodmay further comprise instructions that, when executed by the processor,cause the location determiner to: find intersection points between thefirst circular geometrical object and the second circular geometricalobject; and determine which, if any, of the intersection points that canbe applied to restrict where the mobile device is located, and, whenapplicable, reduce the size of the mobile device region based on the atleast one applicable intersection point.

According to a third aspect, it is provided a computer program fordetermining a location of a mobile device. The computer programcomprises computer program code which, when run on a location determinercauses the location determiner to: obtain a first maximum distancebetween a first anchor point and the mobile device; generate a firstcircular geometrical object, centred on the first anchor point, having aradius based on the first maximum distance; generate a first polytopeencompassing the first circular geometric object, wherein all angles ofthe first polytope are right angles; obtain a second maximum distancebetween a second anchor point and the mobile device; generate a secondcircular geometrical object, centred on the second anchor point, havingradius based on the second maximum distance; generate a second polytopeencompassing the second circular geometric object, wherein all angles ofthe second polytope are right angles; find a mobile device region as anoverlap between the first polytope and the second polytope; anddetermine that the mobile device is located within the mobile deviceregion.

According to a fourth aspect, it is provided a computer program productcomprising a computer program according to the third aspect and acomputer readable means on which the computer program is stored.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating how small inaccuracies oflocation determination can have crucial effects for securityapplications;

FIG. 2 is a schematic diagram illustrating determination of a mobiledevice region with great certainty in two dimensions according to oneembodiment;

FIG. 3 is a schematic diagram illustrating the determination of a mobiledevice region with greater accuracy by exploiting intersection points;

FIG. 4 is a schematic diagram illustrating determination of a mobiledevice region with great certainty in three dimensions according to oneembodiment;

FIG. 5 is a flow chart illustrating a method for determining a locationof a mobile device;

FIG. 6 is a schematic diagram illustrating how intersection pointslocation in relation to anchor points can limit the mobile deviceregion, when the first anchor point is above the second anchor point;

FIG. 7 is a schematic diagram illustrating how intersection pointslocation in relation to anchor points can limit the mobile deviceregion, when the first anchor point is below the second anchor point;

FIG. 8 is a schematic diagram illustrating components of the locationdeterminer of FIG. 1; and

FIG. 9 shows one example of a computer program product 90 comprisingcomputer readable means.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided byway of example so that this disclosure will be thorough and complete,and will fully convey the scope of the invention to those skilled in theart. Like numbers refer to like elements throughout the description.

Embodiments presented herein exploit the fact that a mobile devicecannot be located outside a maximum distance from a measurement point.The measurements define circles (in two dimensions) or spheres (in threedimensions), which, where they overlap contain the mobile device.Additionally, in order to save resource usage, the circles are replacedby polytopes (typically square/cube), for which the overlap can becalculated more efficiently. Significantly, the mobile device can beguaranteed to be located within the overlapping area, which can beexploited e.g. to conclusively determine when a mobile device is withina target area or not.

FIG. 1 is a schematic diagram illustrating how small inaccuracies oflocation determination can have crucial effects for securityapplications.

A target region 9 can e.g. be a building or part of a building, such asa flat or an office. There are here a first anchor point boa and asecond anchor point 10 b. At the first anchor point, there is a distancemeasurement device which can measure the distance to a mobile device 2.The distance can e.g. be measured using Time of Flight (ToF). With ToF,the distance to the mobile device is measured by the time it takes for aradio signal to travel to the mobile device 2 and/or back to thedistance measurement device.

A location determiner 1 is used to determine the location of the mobiledevice 2. The location determiner 1 can be a stand-alone device, or itcan be housed in another device, such as a device at the first anchorpoint boa or the second anchor point 10 b for measuring distance. Whenthe location determiner is implemented in a battery powered device, itis of utmost importance to conserve resource usage for the locationdetermination.

When the distance measurements are used to determine whether the mobiledevice 2 is inside or outside the target region 9, small differences canhave serious effects.

In the prior art, the location of the mobile device can be determined bymeasuring the distance from the two points 10 a, 10 b and determiningthe mobile device to be located at a point satisfying both distancemeasurements. The accuracy is not perfect, whereby there is a margin oferror region 3 around the estimated location. When the margin of errorregion 3 is both inside and outside the target region, it is impossiblefor the location determiner 1 to determine whether the mobile device 2is inside or outside the target region 9. Moreover, even if the mobiledevice 2 most likely is located within the margin of error region 3,this cannot be guaranteed.

However, using embodiments presented herein, the location determiner candetermine the location of a mobile device region in which it can beguaranteed that the mobile device is located, and this is performed in aresource efficient manner.

FIG. 2 is a schematic diagram illustrating determination of a mobiledevice region with great certainty in two dimensions according to oneembodiment. There is here a first anchor point boa from which it can bemeasured a distance to a mobile device. For instance, ToF can be used,the propagation time of a radio signal is used to determine a firstmaximum distance 11 a between the first anchor point boa and the mobiledevice 2. When e.g. ToF is used, the first maximum distance 11 a can bedetermined on the speed of light. While a delay can be introduced byprocessing, etc., this would only be reflected in that the mobile deviceis closer than indicated by the first maximum distance 11 a. The onlyway that the mobile device could be located outside the first maximumdistance 11 a if signal speeds greater than the speed of light isachieved, which is not possible. A first circular geometrical object 12a is generated based on the first maximum distance 11 a, such that thefirst circular geometrical object 12 a is centred on the first anchorpoint boa and has a radius which is the first maximum distance 11 a.Since the scenario of FIG. 2 is in two dimensions, the first circulargeometric object 12 a is here a circle.

Based on the first maximum distance 11 a, it can be safely guaranteedthat the mobile device 2 is within the first circular geometrical object12 a.

Analogously, is can be guaranteed that the mobile device 2 is within asecond circular geometrical object 12 b, which is generated inaccordance with how the first circular geometric object 12 a isgenerated.

By combining the first circular geometrical object 12 a and the secondcircular geometrical object 12 b, it could be determined that the mobiledevice 2 is located within a section where the circular geometricalobjects 12 a, 12 b overlap. However, determining this section iscomputationally expensive, especially if more maximum distances andcorresponding circular geometrical objects are applied.

According to embodiments herein, the location of the mobile device isdetermined in a more computationally efficient manner by additionalsimplifications. A first polytope 13 a is generated which encompassesthe first circular geometric object 12 a. All angles of the firstpolytope 13 a are right angles. Since the scenario of FIG. 2 is in twodimensions, the first polytope 13 a is a, or more specifically a square.More specifically, when the size of the first polytope 13 a is minimalin size, but still encompassing the first circular geometric object 12a, the first polytope 13 a is a square. The same procedure is repeatedfor the second circular geometric object 12 b, resulting in a secondpolytope 13 b being here a rectangle or a square.

Since the first and second circular geometrical objects 12 a-b arerespectively encompassed by the first and second polytopes 13 a-b, anoverlap 16 between the first and second polytopes 13 a-b necessarilycontains the overlap between the first and second circular geometricalobjects 12 a-b. In other words, the mobile device 2 must be locatedwithin the overlap 16 between the first and second polytopes 13 a-b.

The calculation of the rectangular (in two dimensions) overlap 16between the two polytopes 13 a-b is very computationally efficient.

The implication of this is significant, since when the overlap 16between the first and second polytopes 13 a-b is located e.g. within atarget space (9 of FIG. 1), it can be deduced that the mobile device 2must also be located within the target space, which relies on the speedof light for radio signals not being exceeded.

More points and maximum distances can be applied to reduce the size ofthe overlap 16, and thus the accuracy of the location determination.

In other words, the maximum distances 11 a, 11 b are used in acomputationally effective way to determine a mobile device region 15 inwhich the mobile device must be located.

FIG. 3 is a schematic diagram illustrating the determination of a mobiledevice region with greater accuracy by exploiting intersection points.

The scenario of FIG. 3 is similar to that of FIG. 2, but here, a firstintersection point 17 a and a second intersection point 17 b areindicated. The intersection points 17 a, 17 b are where the firstcircular geometrical shape 12 a and the second circular geometricalshape 12 b intersect.

The intersection points 17 a, 17 b are used to reduce the size of themobile device region 15, where, as for FIG. 2, must contain the mobiledevice 2.

Looking at the second intersection point 17 b, the entire overlapbetween the circular geometrical objects is below the secondintersection point 17 b. Hence, the mobile device 2 must be locatedbelow the second intersection point 17 b. This knowledge is used toreduce the mobile device region 15 by reducing the height of the mobiledevice region 15 to the second intersection point 17 b.

By reducing the size of the mobile device region 15, the accuracy isimproved, since there is a smaller mobile device region 15 in which themobile device 2 is known to be located. Nevertheless, the certainty thatthe mobile device is in the mobile device region 15 is not compromisedwith this procedure.

FIG. 4 is a schematic diagram illustrating determination of a mobiledevice region with great certainty in three dimensions according to oneembodiment. In other words, the embodiments presented and describedabove can be expanded to three dimensions.

When applied in three dimensions, the first polytope 13 a and the secondpolytope are cubes or cuboids. The overlapping region 16 between thefirst polytope 13 a and the second polytope 13 b is in the shape of acuboid or a cube. Hence, the mobile device region 15 is also in theshape of a cuboid or a cube.

While not shown in FIG. 4, the first circular geometrical object 12 aand the second circular geometrical object 12 b are here both in theshape of a sphere.

FIG. 5 is a flow chart illustrating a method for determining a locationof a mobile device. The method is performed in a location determiner.

In an obtain 1^(st) max distance step 40, the location determinerobtains a first maximum distance between a first anchor point and themobile device.

In a generate 1^(st) circular object step 42, the location determinergenerates a first circular geometrical object. The first circulargeometrical object is centred on the first anchor point and has a radiuswhich is based on the first maximum distance. When applied in twodimensions, the first circular geometrical object is a circle and whenapplied in three dimensions, the first circular geometrical object is asphere.

In a generate 1^(st) polytope step 44, the location determiner generatesa first polytope encompassing the first circular geometric object. Allangles of the first polytope are right angles. When applied in twodimensions, the polytope is a rectangle or square, and when applied inthree dimensions, the polytope is a cuboid or cube.

In one embodiment, each side of the first polytope touches theencompassed first circular geometrical object.

In an obtain 2^(nd) max distance step 46, the location determinerobtains a second maximum distance between a second anchor point and themobile device. It is to be noted that this step is performed close intime to step 40 to prevent miscalculations due to the mobile devicemoving between obtaining the different distances. For instance, step 40and 46 can be performed within a threshold time of each other.

In a generate 2^(nd) circular object step 48, the location determinergenerates a second circular geometrical object. The second circulargeometrical object is centred on the second anchor point and has aradius which is based on the second maximum distance. When applied intwo dimensions, the second circular geometrical object is a circle andwhen applied in three dimensions, the second circular geometrical objectis a sphere.

In a generate 2^(nd) polytope step 50, the location determinergenerating a second polytope encompassing the second circular geometricobject. All angles of the second polytope are right angles. When appliedin two dimensions, the polytope is a rectangle or square, and whenapplied in three dimensions, the polytope is a cuboid or cube.

In one embodiment, each side of the second polytope touches theencompassed second circular geometrical object.

In one embodiment, at least one side of the first polytope and at leastone side of the second polytope are parallel. For instance, the sides ofthe first polytope and the second polytope can be aligned with axes of aCartesian coordinate system. This significantly simplifies calculationof overlap.

In an optional obtain 3^(rd) max distance step 56, the locationdeterminer obtains a third maximum distance between a third anchor pointand the mobile device.

In an optional generate 3^(rd) circular object step 58, the locationdeterminer generates a third circular geometrical object. The thirdcircular geometrical object is centred on the third anchor point and hasa radius based on the third maximum distance. When applied in twodimensions, the third circular geometrical object is a circle and whenapplied in three dimensions, the third circular geometrical object is asphere.

In an optional generate 3^(rd) polytope step 59, the location determinergenerates a third polytope encompassing the third circular geometricobject. All angles of the third polytope are right angles. When appliedin two dimensions, the polytope is a rectangle or square, and whenapplied in three dimensions, the polytope is a cuboid or cube.

It is to be noted that steps 56, 58 and 59 can optionally be repeatedrespectively for an arbitrary number of anchor points, and thus maximumdistances, circular objects and polytopes, i.e. for a fourth anchorpoint, a fifth anchor point, etc. More anchor points result in a smallermobile device region, and thus greater accuracy.

In an optional find intersections step 51, the location determinerfinds, for the two-dimensional case, intersection points (see 17 a-b ofFIG. 3) between the first circular geometrical object and the secondcircular geometrical object.

Intersections can have the ability to, but do not need to, contribute toreducing the size of the mobile device region.

Intersection points 17 are now explained a bit further with reference toFIG. 6, where intersection points 17 are illustrated in relation to thefirst anchor point boa and the second anchor point bob. Differentpotential sections 19 a-j are shown where intersection points 17 canoccur, based mainly on a combination of quadrants of the two anchorpoints. When applied as explained below, the mobile device region can bereduced by one or two straight lines, reflecting that the mobile deviceis to the right or left, and/or above or below the intersection point17.

The relations illustrated in FIG. 6 can be used to determine whether anintersection point 17 can contribute to reducing the size of the mobiledevice region.

When the intersection point 17 is located in the same quadrants of boththe first anchor point and the second anchor point, see the firstsection 19 c, the third section 19 c, the eighth section 19 h, and thetenth section 19 j, the intersection point 17 does not contribute.

In other words, when the intersection point 17 is located in the thirdsection 19 c, which is defined by the first quadrant of the first anchorpoint boa and the first quadrant of the second anchor point 10 b, theintersection point 17 does not contribute. When the intersection point17 is located in the first section 19 a, which is defined by the secondquadrant of the first anchor point boa and the second quadrant of thesecond anchor point 10 b, the intersection point 17 does not contribute.When the intersection point 17 is located in the eighth section 19 h,which is defined by the third quadrant of the first anchor point boa andthe third quadrant of the second anchor point 10 b, the intersectionpoint 17 does not contribute. When the intersection point 17 is locatedin the tenth section 19 j, which is defined by the fourth quadrant ofthe first anchor point boa and the fourth quadrant of the second anchorpoint 10 b, the intersection point 17 does not contribute.

When the intersection point 17 is located in the second section 19 b,which is defined by the first quadrant of the first anchor point boa andthe second quadrant of the second anchor point 10 b, the mobile deviceis below the intersection point 17.

When the intersection point 17 is located in the fourth section 19 d,which is defined by the third quadrant of the first anchor point boa andthe second quadrant of the second anchor point 10 b, the mobile deviceis to the right of the intersection point 17.

When the intersection point 17 is located in the ninth section 19 i,which is defined by the fourth quadrant of the first anchor point boaand the third quadrant of the second anchor point 10 b, the mobiledevice is above the intersection point 17.

When the intersection point 17 is located in the seventh section 19 g,which is defined by the fourth quadrant of the first anchor point boaand the first quadrant of the second anchor point 10 b, the mobiledevice is to the left of the intersection point 17.

When the intersection point 17 is located in the fourth quadrant of thefirst anchor point boa and the second quadrant of the second anchorpoint 10 b, the intersection point can be either in the fifth section 19e or the sixth section 19 f. The distinction between the fifth section19 e and the sixth section 19 f is based on whether the intersectionpoint 17 is above (for the fifth section 19 e) or below (for the sixthsection 19 f) a straight line 20 between the first anchor point boa andthe second anchor point 10 b.

When the intersection point 17 is located in the fifth section 19 e, themobile device is both below and to the left of the intersection point17. When the intersection point 17 is located in the sixth section 19 f,of the intersection point.

For the three-dimensional case, the location determiner findsintersecting lines between the first circular geometrical object and thesecond circular geometrical object.

In an optional determine restriction(s) based on intersections step 52,the location determiner determines which, if any, of the intersectionpoints that can be applied to restrict where the mobile device islocated.

In a find mobile device region step 53, the location determiner finds amobile device region as an overlap between the first polytope and thesecond polytope.

When there are three polytopes, this step comprises finding the overlapbetween the first polytope, the second polytope and the third polytope.

When applicable, the step of finding the mobile device region comprisesreducing the size of the mobile device region based on the at least oneapplicable intersection point.

In determine mobile device location step 54, the location determinerdetermines that the mobile device is located within the mobile deviceregion. In other words, the location is not here a specific point, but aregion.

In an optional determine whether m.d. (mobile device) in target regionstep 55, the location determiner determines that the mobile device islocated within a target region when the target region fully encompassedthe mobile device region.

FIG. 8 is a schematic diagram illustrating components of the locationdeterminer 1 of FIG. 1. It is to be noted that one or more of thementioned components can be shared with a host device. The locationdeterminer 1 can be battery powered, whereby resources are very limitedin order to preserve battery life. A processor 60 is provided using anycombination of one or more of a suitable central processing unit (CPU),multiprocessor, microcontroller, digital signal processor (DSP), etc.,capable of executing software instructions 67 stored in a memory 64,which can thus be a computer program product.

The processor 60 could alternatively be implemented using an applicationspecific integrated circuit (ASIC), field programmable gate array(FPGA), etc. The processor 60 can be configured to execute the methoddescribed with reference to FIG. 5 above.

The memory 64 can be any combination of random-access memory (RAM)and/or read-only memory (ROM). The memory 64 also comprises persistentstorage, which, for example, can be any single one or combination ofmagnetic memory, optical memory, solid-state memory or even remotelymounted memory.

A data memory 66 is also provided for reading and/or storing data duringexecution of software instructions in the processor 60. The data memory66 can be any combination of RAM and/or ROM.

The location determiner 1 further comprises an I/O interface 62 forcommunicating with other external entities. Optionally, the I/Ointerface 62 also includes a user interface.

Other components of the location determiner 1 are omitted in order notto obscure the concepts presented herein.

FIG. 9 shows one example of a computer program product 90 comprisingcomputer readable means. On this computer readable means, a computerprogram 91 can be stored, which computer program can cause a processorto execute a method according to embodiments described herein. In thisexample, the computer program product is an optical disc, such as a CD(compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. Asexplained above, the computer program product could also be embodied ina memory of a device, such as the computer program product 64 of FIG. 8.While the computer program 91 is here schematically shown as a track onthe depicted optical disk, the computer program can be stored in any waywhich is suitable for the computer program product, such as a removablesolid-state memory, e.g. a Universal Serial Bus (USB) drive.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope of the invention, as defined by the appendedpatent claims.

What is claimed is:
 1. A method for determining a location of a mobiledevice, the method being performed in a location determiner andcomprising: obtaining a first maximum distance between a first anchorpoint and the mobile device; generating a first circular geometricalobject centred on the first anchor point, having a radius based on thefirst maximum distance; generating a first polytope encompassing thefirst circular geometric object, wherein all angles of the firstpolytope are right angles; obtaining a second maximum distance between asecond anchor point and the mobile device; generating a second circulargeometrical object, centred on the second anchor point, having radiusbased on the second maximum distance; generating a second polytopeencompassing the second circular geometric object, wherein all angles ofthe second polytope are right angles; finding a mobile device region asan overlap between the first polytope and the second polytope; anddetermining that the mobile device is located within the mobile deviceregion.
 2. The method according to claim 1, wherein at least one side ofthe first polytope and at least one side of the second polytope areparallel.
 3. The method according to claim 2, wherein the sides of thefirst polytope and the second polytope are aligned with axes of aCartesian coordinate system.
 4. The method according to claim 1, furthercomprising: determining that the mobile device is located within atarget region then the target region fully encompassed the mobile deviceregion.
 5. The method according to claim 1, wherein each side of thefirst polytope ouches the encompassed first circular geometrical objectand each side of the second polytope touches the encompassed secondcircular geometrical object.
 6. The method according to claim 1, whereinthe geometrical objects are two dimensional objects, and the methodfurther comprises: finding intersection points between the firstcircular geometrical object and the second circular geometrical object;and determining which, if any, of the intersection points that can beapplied to restrict where the mobile device is located, and, whenapplicable, finding the mobile device region comprises reducing the sizeof the mobile device region based on the at least one applicableintersection point.
 7. The method according to claim 1, furthercomprising: obtaining a third maximum distance between a third anchorpoint and the mobile device, generating a third circular geometricalobject, centred on the third anchor point, having a radius based on thethird maximum distance; and generating a third polytope encompassing thethird circular geometric object, wherein all angles of the thirdpolytope are right angles; wherein finding an overlap comprises findingthe overlap between the first polytope, the second polytope and thethird polytope.
 8. A location determiner for determining a location of amobile device, the location determiner comprising: a processor; and amemory storing instructions that, when executed by the processor, causethe location determiner to: obtain a first maximum distance between afirst anchor point and the mobile device generate a first circulargeometrical object, centred on the first anchor point, having a radiusbased on the first maximum distance; generate a first polytopeencompassing the first circular geometric object, wherein all angles ofthe first polytope are right angles; obtain a second maximum distancebetween a second anchor point and the mobile device; generate a secondcircular geometrical object, centred on the second anchor point, havingradius based on the second maximum distance; generate a second polytopeencompassing the second circular geometric object, wherein all angles ofthe second polytope are right angles; find a mobile device region as anoverlap between the first polytope and the second polytope; anddetermine that the mobile device is located within the mobile deviceregion.
 9. The location determiner according to claim 8, wherein atleast one side of the first polytope and at least one side of the secondpolytope are parallel.
 10. The location determiner according to claim 9,wherein the sides of the first polytope and the second polytope arealigned with axes of a Cartesian coordinate system.
 11. The locationdeterminer according to claim 8, further comprising instructions that,when executed by the processor, cause the location determiner to:determine that the mobile device is located within a target region whenthe target region fully encompassed the mobile device region.
 12. Thelocation determiner according to claim 8, wherein each side of the firstpolytope touches the encompassed first circular geometrical object andeach side of the second polytope touches the encompassed second circulargeometrical object.
 13. The location determiner according to claim 8,wherein the geometrical objects are two dimensional objects, and themethod further comprises instructions that, when executed by theprocessor, cause the location determiner to: find intersection pointsbetween the first circular geometrical object and the second circulargeometrical object; and determine which, if any, of the intersectionpoints that can be applied to restrict where the mobile device islocated, and, when applicable, reduce the size of the mobile deviceregion based on the at least one applicable intersection point.
 14. Acomputer program for determining a location of a mobile device, thecomputer program comprising computer program code which, when run on alocation determiner causes the location determiner to: obtain a firstmaximum distance between a first anchor point the mobile device;generate a first circular geometrical object, centred on the firstanchor point, having a radius based on the first maximum distance;generate a first polytope encompassing the first circular geometricobject, wherein all angles of the first polytope are right angles;obtain a second maximum distance between a second anchor point and themobile device; generate a second circular geometrical object, centred onthe second anchor point, having radius based on the second maximumdistance; generate a second polytope encompassing the second circulargeometric object, wherein all angles of the second polytope are rightangles; find a mobile device region as an overlap between the firstpolytope and the second polytope; and determine that the mobile deviceis located within the mobile device region.
 15. A computer programproduct comprising a computer program according to claim 14 and acomputer readable means on which the computer program is stored.